The present invention relates to fasteners in general, and, more in particular, to fasteners with a prevailing torque lock.
A prevailing torque lock on a threaded fastener prevents unwanted rotation of a female threaded element with respect to a male threaded element. (These two elements are called many things, for example: nuts, collars, pins and screws, but they will most often be called "nuts" and "bolts" in this specification.) A number of prevailing torque thread locks are known. Some employ a plastic insert in the thread of the nut to increase the friction between the nut thread and the thread of a bolt. Others rely on deformed nut threads, produced, for example, by crimping the threads.
When a nut must apply a closely controlled load, a prevailing torque thread lock is an excellent way of accommodating close load tolerance and maintaining the load in service.
When either the nut or the bolt bears on a rotating work face, prevailing torque thread locks have not generally been used because the provision for a barrier washer used to isolate the rotation can adversely affect the thread lock performance. An automobile spindle asembly is an example where prevailing torque thread locks have not been used.
In a typical automobile spindle assembly, an axle of the spindle, through wheel bearing sets, mounts a wheel for rotation. An adjusting nut on a threaded shank of the spindle holds the bearing sets and wheel on the axle. The adjusting nut loads the bearing sets a predetermined amount correlated to the torque applied to the nut during its installation. For optimum bearing life, the amount of bearing load must be held to within tight limits. Typically, the installer tightens the nut down to a desired tightness, say, 25 foot pounds of torque. The installer then backs the nut off about half a turn and then tightens the adjusting nut to some desired valve, say, about 15 inch pounds torque, and then installs a retainer, if necessary, and a cotter pin safety.
In such an arrangement, a transverse hole in the spindle receives the cotter pin that locks the nut against rotation by passing through castellation slots of the nut or of a nut retainer used with the nut.
The accuracy of loading on the wheel bearings with this technique depends entirely on registration of castellation slots of the adjusting nut or retainer with the cotter pin hole in the spindle. The variation in bearing loading dictated by this registration requirement often reduces bearing performance.
Since the wheel rotates on the spindle, its rotation can apply an unloosening or tightening torque, depending on the direction of rotation, to the adjusting nut. To prevent this, a barrier washer keyed to the spindle so that the washer does not rotate isolates the nut from wheel rotation. Keying may be done by a keyway in the spindle and a key tab of the washer received in the keyway. Keying of the washer may also be done by one or more flats on the spindle that cooperate with mating flats of the washer to keep the latter from rotating.
The requirement for the keyway slot in the spindle rules out many prevailing torque type thread locks and forces the use of the castellated nut method of locking the nut to the spindle, even with its many disadvantages. This is so because the edges of the keyway can abraid and ruin the plastic in plastic insert type prevailing torque lock nuts; and, at least in single crimp style prevailing lock nuts, the crimp can register with the slot permitting some rotation of the nut with a concomitant unacceptable loosening or tightening of the wheel.
With a spindle having flats instead of a keyway to keep the washer from rotating, the same problems occur with prevailing torque thread locks: The edges of the flats abraid the plastic of plastic-type prevailing torque lock nuts; and the flats can register with crimps of the nuts on that type of prevailing torque thread lock and permit some loosening or tightenting of the wheel.
Another disadvantage of castellated nut safetying, in addition to less than optimum bearing loading, is that the reliability of the safety depends on proper installation of the cotter pin. Proper installation requires rather careful hand bending of the prongs of the cotter pin; one prong must be bent over the bolt end and not extend beyond the bolt diameter, requiring cutting the prong if necessary; the second prong must be bent down against a wrenching flat of the nut and cannot rest against the barrier washer surface, again sometimes requiring cutting of the prong. If an optional prong wrap around method is used, the prongs cannot extend outward from the wrenching surfaces of the nut. With either method, all prongs must be bent leaving a reasonable radius at the bends in order to avoid prong breakage. Correct cottering becomes very demanding in inaccessible locations and in cold environments.
Self-tapping male threaded fasteners of arcuate triangular cross-sectional or tri-lobular shapes are well know. H. F. Phipard, Jr., describes a special form of such fasteners in U.S. Pat. Nos. 3,249,142 and 3,918,345, among other patents. The fasteners described in the Phipard patents have three, 120.degree. spaced apart lobes or tips of major diameter and intermediate sides of minor diameter; the diameter measured across the shape anywhere around 360.degree. is the same regardless of where taken because the rise to each of the lobe tips and the fall to each side's minimum radius are in phase across connecting diameters. Because two parallel flat surfaces engaged with a tri-lobular surface remain engaged regardless of the rotational position of the tri-lobular surface and without varying the distance between the surfaces, a thread can easily be rolled onto the surface with standard, flat thread dies if the surface has a convex curvature that is not too flat.
In the U.S. Pat. No. 3,249,142, Phipard says that the self-tapping screw he describes provides a high degree of back-off torque after many loosening cycles in a prevailing torque thread lock type fastener, which means that the thread lock remains good after many removals and reinstallations of the screw. The thread lock results from interference between the lobes of the screw and the walls of the female fastener cooperating with it.
Self-tapping screws, of course, permanently deform the workpiece tapped into, sometimes by cutting, sometimes by swaging. The workpieces must be soft for this to be done. Further, the conical lead of the thread has no utility other than in piloting the balance of the thread into the hole; the lead-in thread convolutions carry no load after screw setting.
While the two Phipard patents relate in particular to self-tapping screws that have threaded conical lead-in tips of substantial length, a lobular form bolt without the conical lead is described in U.S. Pat. No. 3,434,168 to Bonacci. Bonacci does not describe a true tri-lobular form.
A prevailing torque thread lock fastener with a different philosophy is shown in U.S. Pat. No. 4,381,163 to Witte. Witte shows a nut having a thin walled section with a thread pitch diameter smaller than in the base of the nut to produce a prevailing torque interference lock with a bolt. To enhance the lock, Witte plastically deforms the thin walled section with three equiangular crimps. When the nut is installed on the bolt, the crimped section tends to round out under the force of the circular bolt thread. Witte's bolt has a circular in cross-section thread.
In Witte, the bolt always acts on the nut only in the crimps and on the same nut material in the crimps. With reuse, Witte's nut wears out at the crimps because the same material always experiences stress.
U.S. Pat. No. 2,349,513 to Mortus describes a similar and well known approach. Mortus describes a nut having fingers separated by longitudinal slots; the fingers are squeezed in to reduce the diameter of the nut thread. The approach in Mortus does not use an acircular thread perimeter. The problems with the Mortus approach include a comparatively high cost of manufacture, poor tension strength of the nut in the slotted section, and a tendency of the edges of the slots to gall the male thread or its coating.